Transformers are used to change the characteristics of an alternating current, for example, by changing the voltage magnitude, current magnitude, or phase angle of the alternating current. Transformers include a first multi-turn coil winding in close proximity to a second multi-turn coil winding. The alternating current in the first coil induces magnetic flux in a magnetic core. The magnetic flux then induces an alternating current, with new electrical characteristics, in the second coil.
Core-form transformers are known in the art. For a three-phase core-form transformer, the core includes three legs connected by yokes. Each of the core legs has a core-form winding positioned around it. The core and windings are then placed in a tank which includes openings for electrical connections to the windings. The openings are also used to position cooling equipment.
Operation of a transformer results in the generation of heat. A variety of techniques are used to remove heat from a transformer. A low power transformer may be self-cooling, while a medium power transformer may require a fan for cooling. Large capacity power transformers generally rely upon liquid cooling. That is, liquid is forced through the windings to remove heat, and the liquid is then cooled at a heat exchanger. Oil is a typical cooling liquid used in transformers. The heat exchanger is typically a fan-cooled finned tube. A pump circulates the oil through the transformer and heat exchanger.
FIG. 1 is a perspective view of a section of a prior art core-form transformer winding 20. The winding 20 includes a winding tube 22. A conductor is wound around the tube 22 to form coil segments 24A and 24B, which form a coil section 26. The coil segments 24A and 24B are radially displaced from one another by duct spacers 28. The duct spacers 28 result in internal vertical ducts 30 along the length of the transformer winding 20. The winding 20 also includes radial spacers 32. The radial spacers 32 vertically separate each coil section 26.
In a complete winding, a new coil segment 24C (not shown) would be formed over duct spacers 28C. In a similar manner, a number of coil segments would be formed.
FIG. 2 is an enlarged perspective view of a segment of the core-form transformer winding of FIG. 1. FIG. 2 depicts coil segments 24A and 24B being radially displaced from one another by duct spacers 28. The coil section 26A is vertically separated from coil section 26B by a radial spacer 32.
FIG. 2 only depicts two coil segments 24A and 24B, an actual winding would have several more coil segments, with duct spacers 28 between each segment. Thus, it should be appreciated that a number of coil segments will be interposed between coil segment 24B and the outer end 33 of the radial spacer 32. The outer end 33 of the radial spacer 32 includes a notch 35 which receives an outer vertical spacer 37. The outer vertical spacer 37 extends the entire length of the winding 20. An inner vertical spacer 39 is positioned between the tube 22 and the coil segment 24A. The inner vertical spacer 39 extends the entire length of the winding.
FIG. 3 is a cross-sectional view of the transformer winding 20 taken along the line 3--3 of FIG. 1. The figure reflects that the transformer winding 20 includes a number of coil segments 24A, 24B, and additional segments indicated by the dots. These coil segments form a number of coil sections 26A, 26B, etc. Each coil segment 24 is formed from a conductor 41 which is wound around the winding tube 22. The duct spacers 28 (not shown) between coil segments result in internal vertical ducts 30. Similarly, the radial spacers (not shown) result in radial ducts 45. The inner vertical spacer (not shown) forms an inner vertical duct 49 and the outer vertical spacer (not shown) forms an outer vertical duct 51. The winding 20 is enclosed by an outer wrap 52.
A liquid coolant is forced from the bottom of the winding 20 (the bottom of the page), to the top of the winding. To obtain favorable flow conditions within the winding, prior art windings include one or more radial barriers 47. By alternately blocking the flow path of the coolant, the barriers force the coolant to flow back and forth, or in a zig-zag fashion, as the coolant is forced from the bottom to the top of the winding 20.
There are a number of problems associated with the radial barriers 47 used in the prior art. First, the hydrodynamics of the back and forth flow prevents uniform distribution to the radial ducts 45. This phenomenon results from the fact that the coolant takes the path of least resistance, which means that the coolant will travel through the inner vertical duct 49 and the outer vertical duct 51, instead of altering its direction through the radial ducts 45. To remedy this problem, it is necessary to increase the pressure of the coolant entering the winding. Thus, there is high pressure at the bottom of the winding, this pressure decreases through frictional effects within the winding until the pressure at the top of the winding is very low. Thus, there is a problematic pressure drop from the bottom of the winding to the top of the winding. A high pressure drop reduces oil circulation and thereby increases the winding temperature.
It is expensive to install the prior art radial barriers 47. Installation requires the hand fitting of washer-shaped pieces into a large number of notches formed on the radial spacers 32. The notches require precise machining.